1. Field of the Invention
This invention relates to novel p-hydroxycinnamamide derivatives and also to a melanin inhibitor comprising the same as an active component.
2. Description of the Prior Art
The color of the skin is chiefly influenced by the amount of melanin in the epidermis. This melanin is invariably produced within the melanocytes and is released toward surrounding corneocytes, and thus falls off as the cornification proceeds, so that the density within the epidermis is maintained constant. Accordingly, the skin color becomes uniform and little change in the color is recognized through a year. However, the melanin production mechanism is promoted by the action of UV rays and a melanocyte stimulating hormone (MSH) or by ageing, and the skin is clinically melanized. If the melanization takes place locally, a clear difference from a surrounding normal skin appears and may result in pigmental spots such as freckles or chloasma.
These spots, freckles, chloasma, and pigmentation after sunburn tend to occur and increase with aging or become more difficult to disappear, and have been one of typical skin troubles of aged persons.
Accordingly, there is a strong demand for a medicine which is able to return the pigmental spots to the surrounding normal skin color.
For this purpose, a variety of medicines have been proposed and some have been in use. For instance, peroxides are considered to have the action of bleaching the produced melanin. Attempts have been made to use hydrogen peroxide, benzoyl peroxide, and the like. However, these compounds are very unstable and have not been recognized to show the practical effect of preventing the pigmental deposition. In recent years, cosmetics containing vitamin C (L-ascorbic acid) having good reducing ability have been used, but the vitamin C is not so stable and does rarely show an appreciable effect. In Europe and the United States of America, hydroquinone and derivatives thereof, and various catechols have been used to cure spots or as medicines for bleaching of colored person's skin. These compounds however, leave much to be desired from the standpoint of safety because they may sometimes cause irritation and allergy. Besides, in some cases, they produce vitiligo thus being difficult for use as a melanin inhibitor. Aside from the above, a variety of melanin inhibitors (or bleaching cosmetics) have been reported but few are safe and show a substantial effect of improving the pigmental deposition.
The present inventors proposed in our Japanese patent application Laid-open No. 60-190713 p-hydroxycinnamamide derivatives of the following formula (a) ##STR2## in which R represents a hydrogen atom, and an alkyl, cycloalkyl or alkenyl group having from 1 to 24 carbon atoms, these derivatives having the melanin inhibiting action.
However, these compounds were not satisfactory with respect to the miscibility with other ingredients when formulated in cosmetics and the percutaneous absorptivity.